This invention relates to a temperature-controlled fluid friction coupling for intermittently driving a cooling fan of an internal combustion engine.
In a typical design of a temperature-controlled fluid friction coupling, such as U.S. Pat. No. 4,662,495--Brunken, a housing is rotatably supported on a drive input shaft adapted to connect with an internal combustion engine, and the interior space of the housing is divided into a reservoir chamber and a torque transmitting chamber by a partition. On the drive input shaft a rotor is fixed so as to form mutually opposite shearing surface gaps between the housing and the rotor. A cooling fan is attached to an outer surface of the housing which comprises a front cover and a rear body.
To the outer surface of the front cover, is mounted a temperature sensitive element made of a bimetal which curves in response to a change of ambient temperature. In the partition, a valve opening is provided and a valve lever for opening and closing the valve opening is mounted. The valve lever is initially stressed in the opening direction. Between the bimetal and the valve lever, is disposed an actuating pin which transmits a deformation of the bimetal to the valve lever.
As the bimetal is deformed by a change of the ambient temperature, the valve lever moves toward the same direction through the actuating pin, allowing the valve opening to open or close. Then, viscous fluid flows into or out of the shearing surface gaps to effect or break the torque transmitting performance.
The ambient temperature sensed by the bimetal is generally the temperature of air after having passed through a radiator. For example, at a low temperature below 60 degrees centigrade, the bimetal is kept in a relatively flat shape and the valve lever closes the valve opening. In this situation, viscous fluid is raked out by a dam from the torque transmitting chamber to the reservoir, whereby the fluid-friction coupling is kept in OFF condition.
Conversely, at a high temperature above 60 degrees centigrade, the bimetal curves toward the outer direction of the housing, permitting the free end of the valve lever to separate from a periphery of the valve opening. In this situation in turn, the viscous fluid flows from the reservoir to the torque transmitting chamber, whereby the fluid friction coupling is turned into ON condition.
During the OFF condition, the transmitting torque is relatively small and a fan rotation speed is relatively low. During the ON condition, the transmitting torque goes up and the fan rotation speed also goes up.
However, even in the OFF condition, the fan idling speed tends to rise to a relatively high level, for example 900 rpm in a winter morning. This is well known as "a creeping revolution" caused by a relatively small transmitting torque which is produced by a residual viscous fluid within the shearing surface gaps, as well as produced by a bearing friction between the housing and the drive input shaft.
Originally, the fluid friction coupling was provided for the purpose of lowering a fan idling revolution thereby to reduce a noise, fuel consumption, and an engine warming up time. However, the existing creeping revolution exhibits undesirable drawbacks against the original purposes.